Use of hydraulic motors has led to motors being designed that have a plurality of active cubic capacities in operation. For example, in motors having radial pistons, it is known that either all of the cylinders can be fed periodically with fluid under pressure, which corresponds to the maximum cubic capacity of the motor, or else the feed of fluid under pressure can be interrupted to some only of the cylinders, which corresponds to an intermediate cubic capacity of the motor, which capacity may even be reduced to zero. For those of the cylinders which are momentarily isolated from the supply of fluid under pressure, it is also known that the pistons which are slidably mounted in those cylinders can be "disengaged" by causing said pistons to cease to abut against the cam, thereby preventing friction and premature wear. To this end, in that known technique, a "disengaging" device enables said pistons to be kept in the configuration in which they are retracted inside their respective cylinders. The main drawback with that technique lies in "disengaging" all of the pistons simultaneously without taking account of their relative instantaneous positions relative to the cam. Difficulties are encountered during the "disengagement", and when certain additional and costly precautions are not taken, such difficulties sometimes give rise to interference between the cam and a piston that is not fully retracted inside its cylinder, and to violent crashing. But even greater difficulties exist when the reverse operation is performed, consisting in putting the pistons back into abutment against the cam ("re-engagement").